St. Michael's Hertfordshire Puddingstoneby Michael Laws FGS

Puddingstone.Does any other rock form have a more obvious and descriptive name? It looks just like a pudding.

Puddingstones exist all over the world. They are conglomerates, that is a mixture of fine and coarse grained material: sands and stones cemented together in a matrix of clay, mudstone or chalky limestone. They are found in and adjacent to riverbeds where rocks from a river’s source have been washed down; in flood plains, where seasonal flooding leaves mixed deposits; in glacial moraines, where ice flow has pushed a wall of eroded material in front of it before receding to leave the mixture to cement together; and on some sea shore-lines, where a receding sea level has prevented the erosion of the fine-grained matrix which would otherwise leave only a clean pebble bed.

So, if puddingstones are so common what makes the Hertfordshire Puddingstone so special?

The answer is that it is especially hard, the matrix being made of silica and thus a rock in itself. The peculiarities of the Hertfordshire Puddingstone can easily be seen:

A crack in the St Michael’s Pudddigstone

The coarse-grained element is always of rounded pebbles.

The pebbles are stained with a reddish coating of iron oxide.

The extreme hardness is demonstrated by the fact that a crack will cut straight through the pebbles rather than pass around them. This can be seen in St. Michael’s puddingstone outside Kingsbury Mill.

Because it is hard and has attractive shapes and colours it has been used in jewellery and is particularly valued by some lapidarists. After all there is more diamond in the world than Hertfordshire Puddingstone!It appears as building material and tools dating back to the Romans and before. Although any in Verulamium have been long since looted, examples may be seen in Hertford where the museum has a rubbing stone or perhaps a hand hammer which has been dated to around 3,500 BC. Its hardness made a useful milling tool. The Verulamium Museum contains an example of a Roman quern stone made of puddingstone and a rubbing-stone, or perhaps hammer, dated back to around 3,500 BC. In recent years, construction of the A10 extension near Puckeridge revealed what has been identified as a Stone Age flint and puddingstone quarry, later used by the Romans whose Ermine Street was the forerunner of the A10. There are several myths about the Hertfordshire Puddingstone. Superstitious countrymen called it Breeding Stone or Mother Stone believing it gave birth to pebbles and gathered fruit or children to itself. Some thought that the stones were alive, could move and had feelings and emotions. One myth relates that a block of puddingstone between Thorley and Bishops Stortford turned around when it heard the town clock in Stortford strike midnight. That at Kingsbury Mill is thought to have been dragged from the river Ver. “It will continue to grow as long as it stands.” Puddingstones can be quite suddenly exposed in fields by rainstorms or sharp frost. Because of their mysterious appearance this caused a superstition among farmers that they grew in the ground. They were broken up but continued to appear as, like flints, they can still do today. Flints with holes through them supposedly possessed supernatural powers and were hung as charms against illness. They were hung in barns and on horses’ harnesses to keep away evil spirits especially “Night Hags”, (Macbeth’s “Silent black and midnight hags”), that were thought to get into stables and ride the horses all night, returning them lathered in sweat. This was supposed to be the cause of “sweating sickness.”Puddingstones were used for similar purposes and then called “hagstones”. Blocks of puddingstone were placed on doorsteps, on village greens and in churchyards, to ward off these witches. A parish record of 1662 records how a suspected witch was to be prevented from escaping from her grave. “A hagstone be placed on the coffin for her bodie within be bewitched.”Examples of Hertfordshire Puddingstone can be seen in a band running across the county. Besides the large piece by the mill there are others in the garden of Kingsbury Manor and at the entrance to St. Michael’s Court; to the west at Potters Crouch; and in the east at Standon, Puckeridge, the Hadhams and the Pelhams.So the Hertfordshire Puddingstone is unique among others, but what made it so? Where did it come from and when?We start with the deposition of the chalk that now forms the Chiltern Hills, which took place between 97 and 65 million years ago, the late half of the period named the Cretaceous, from the Latin for chalk. At that time sea covered the greater part of what is now England. It was a relatively calm and warm sea since Europe was then at a latitude of the present day Mediterranean.So the chalk sea was calm and warm and our old friends the dinosaurs still roamed the countryside while the progenitors of their replacements swam these seas and tried to fly between the trees. Then at about 65 million years ago there was a dramatic change as a large asteroid, or maybe a comet, collided with Earth at a point which is now beneath the Yucatan peninsular in Central America. The resulting change in climate probably did for the dinosaurs. The Late Cretaceous chalk gave way to the mudstone and later sandstone of what is called the Tertiary era, subdivided into first the Palaeocene, which lasted some 10 million years, and then the Eocene, which continued for a further 21 million. The Palaeocene/Eocene boundary is a time that we shall be considering again.As the sea receded the land rose and the whole of eastern England tilted slowly towards the east and deposition of the clays and sandstones of the Tertiary ceased except in what is now the North Sea. There was a shore line running across England from the Isle of Wight to northern Norfolk, with rivers flowing off the land that eventually rose to form the Chiltern Hills. These rivers eroded much of the clay and sandstone and some of the upper levels of the chalk and carried the sediment down to the sea and to the shore line across Hertfordshire. With this sediment came flints that settled as pebble beds and together they provided the material for the Puddingstone.The flints were originally formed within the chalk by chemical changes. Silica from tiny marine organisms replaced the carbon in the calcium carbonate of the chalk where sponges and burrowing creatures provided the spaces, hence the erratic shapes of the flints as we know them: knobbly bits of rock or sharp flakes when split. Those that form the Puddingstone, first by clattering along the river beds and then by wave action on the beach, were broken and rolled into smooth pebbles. Soon after arriving on the shore the flint pebbles must have acquired their attractive red coating of iron oxide from that dissolved in the sea. As the sea receded further, the carbonates of the eroded chalk settled around the pebbles and were replaced by silicates that were dissolved in the sea water, in the same way that the flints were originally formed. The action of additional warmth over a few hundred thousand years hardened the silica into the matrix which is little different in its chemical composition from that of the flint pebbles themselves. Sand, clay and chalk make cement, but the Hertfordshire Puddingstone has this special silica cement which makes it particularly hard.So, these processes, the solution of silica in water and its subsequent replacement of carbonates, is accelerated by additional heat. Where did that come from? At the time that we are considering northern Europe was at a latitude similar to that of the present day Mediterranean and a plot of past global average temperatures shows that they were 8-12 C higher than today. Britain thus had a climate more like that of Malaysia!Records show that around the time of transition from Palaeocene to Eocene there was a marked and, in geological terms, quite sudden and short period of even higher temperatures, often referred to as the P/E Thermal Maximum. The cause of this is still a matter of debate. However it was a time of several significant changes around the world. Associated with the separation of America and Europe and the opening of the North Atlantic, there were huge volcanic outpourings taking place between the Shetland and Faeroe islands and a rising plume of magma from the Earth’s interior was creating Iceland. All this would have raised global temperatures. In addition there is strong support for there having been a sudden catastrophic release of the greenhouse gas methane from where it had been trapped in deep rock formations. The release could have been brought about by any of the earth movements that were taking place at that time. If this interpretation is correct we have an explanation for the exceptional hardness of the Hertfordshire Puddingstone. We already know of what and how it was composed; now we can date its age back to the Eocene/Palaeocene Boundary 55 million years ago.With the current predictions of global warming and rising sea level, together with the fears that some geologists have of a fresh release of methane which would bring a further rise in temperature, maybe we shall see the production of a new generation of Hertfordshire Puddingstone from the gravel of the beach at St. Michael’s Bridge. But we shall have to wait few million years!Back